It's exactly this. It seems that the file names for some of the audio files have been changed? When I look at my www folder most of the bgs files have rpgmvo format (now). I think this is the source of the error.
Sorry, I don't mean to sound condenscending, but do you mean "most", or "all"? I don't have any non-rpgmvo files in any of the audio folders. Though I see why I haven't run into the error maybe, as I haven't spent much time in the gym.
I did get similar errors in Ambrosia, where certain filenames were incorrect, can't remember which. But if I recall my workaround, this might work for you (not that it's good to have to do this, but it stopped the errors):
make a copy of the Honeydrop_b_semmo.rpgmvo file in your bgs folder, and simply rename that copy to Honeydrop_b_semenmo.rpgmvo
Do the same for any similar errors you see.
I hope that helps, even if it's a kludge. And by the way, it may not be that the patch introduced the error, but only that the patch properly lets the game trigger an event which includes the error, never hit in the unpatched game.